1. Field of the Invention
The present invention relates to an air passage switching system having a butterfly door for opening and closing an air passage through which air flows. The air passage switching system is suitable for an air outlet mode switching unit for a vehicle air conditioner.
2. Related Art
A conventional air passage switching system for an air conditioner of a vehicle has an air conditioning case for forming an air passage, and a butterfly door for switching the air passage. A rotation shaft for rotating the butterfly door is disposed at side end centers of the door. The butterfly door is rotatably held in the air conditioning case to be rotated by the rotation shaft.
JP-Y2-2505278 and U.S. Pat. No. 4,402,486 respectively propose an air passage switching system in which a butterfly door is air-tightly connected to an air conditioning case around a rotation shaft. In JP-Y2-2505278, a seal member made of an elastic material such as rubber is attached to an outer peripheral portion of the door. The seal member has a brim-like protrusion portion around the rotation shaft of the door, and the protrusion portion of the seal member contacts an inner wall of the case while being pressed, so that the case around the rotation shaft is air-tightly sealed by the seal member. However, in JP-Y2-2505278, when the rotation shaft rotates the door, the protrusion portion of the seal member constantly slides on the inner wall of the case. Further, the seal member is made of an elastic material having a relatively large friction coefficient. Therefore, a sliding friction between the seal member and the case becomes larger, thereby increasing an operation force of the door. Further, due to a dimension tolerance between the door and the case in a longitudinal direction of the rotation shaft, the protrusion portion of the seal member may not air-tightly contact the inner wall of the case, and air may be leaked from the contacting portion between the protrusion portion of the seal member and the inner wall of the case.
Similarly, in U.S. Pat. No. 4,402,486, a seal member made of elastic material such as rubber is attached to an outer peripheral portion of the door. The seal member has a cylindrical portion extending to surround the rotation shaft of the door. A diameter of the cylindrical portion is decreased toward a top end of the rotation shaft. The top end of the cylindrical portion of the seal member, having a smaller diameter, is press-fitted to an outer circumferential surface of a cylindrical protrusion formed on an inner surface of the case, so that the case around the rotation shaft of the door is air-tightly sealed by the seal member. However, in U.S. Pat. No. 4,402,486, when the rotation shaft is rotated, the top end of the cylindrical portion of the seal member slides on the outer circumferential surface of the cylindrical protrusion of the case, thereby increasing the operation force of the door. Further, since the cylindrical portion of the seal member is made of elastic material such as rubber, the cylindrical portion is readily elastically deformed. Therefore, installation performance of the seal member to the outer circumferential surface of the cylindrical protrusion of the case may become difficult.